brew-pull: fix third-party taps

Closes #17588.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
ruby-build 20130129

References #17545.
rbenv-default-gems: 1.0.0 (new formula)

Closes #17545.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
rbenv-default-gems: improve caveats.

References #17545.